An old world's generation can only go so high now. Well because the world hight limit in an old world is, for some reason, 128 blocks. So if you get a tall generation like the new mountains biome, (seed -1894053832), it will be cut off and sometimes there's naturally generated invisible and unbreakable blocks that are not barriers.
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We're tracking this issue in MCPE-18186, so this ticket is being resolved and linked as a duplicate.
That ticket has already been resolved as working as intended, which means this is not considered a bug and won't be fixed. Please do not leave a comment on the linked ticket.
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